The effects of passive leg raising may be detected by the plethysmographic oxygen saturation signal in critically ill patients

Crit Care. 2019 Jan 18;23(1):19. doi: 10.1186/s13054-019-2306-z.


Background: A passive leg raising (PLR) test is positive if the cardiac index (CI) increased by > 10%, but it requires a direct measurement of CI. On the oxygen saturation plethysmographic signal, the perfusion index (PI) is the ratio between the pulsatile and the non-pulsatile portions. We hypothesised that the changes in PI could predict a positive PLR test and thus preload responsiveness in a totally non-invasive way.

Methods: In patients with acute circulatory failure, we measured PI (Radical-7) and CI (PiCCO2) before and during a PLR test and, if decided, before and after volume expansion (500-mL saline).

Results: Three patients were excluded because the plethysmography signal was absent and 3 other ones because it was unstable. Eventually, 72 patients were analysed. In 34 patients with a positive PLR test (increase in CI ≥ 10%), CI and PI increased during PLR by 21 ± 10% and 54 ± 53%, respectively. In the 38 patients with a negative PLR test, PI did not significantly change during PLR. In 26 patients in whom volume expansion was performed, CI and PI increased by 28 ± 14% and 53 ± 63%, respectively. The correlation between the PI and CI changes for all interventions was significant (r = 0.64, p < 0.001). During the PLR test, if PI increased by > 9%, a positive response of CI (≥ 10%) was diagnosed with a sensitivity of 91 (76-98%) and a specificity of 79 (63-90%) (area under the receiver operating characteristics curve 0.89 (0.80-0.95), p < 0.0001).

Conclusion: An increase in PI during PLR by 9% accurately detects a positive response of the PLR test.

Trial registration: ID RCB 2016-A00959-42. Registered 27 June 2016.

Keywords: Cardiac index; Fluid responsiveness; Oxygen saturation; Perfusion index; Volume expansion.

MeSH terms

  • Aged
  • Cardiac Output / physiology
  • Critical Illness
  • Female
  • Hemodynamics / physiology*
  • Humans
  • Leg / blood supply
  • Leg / physiopathology
  • Linear Models
  • Male
  • Middle Aged
  • Monitoring, Physiologic / methods
  • Monitoring, Physiologic / trends
  • Oxygen / analysis*
  • Oxygen / blood
  • Plethysmography / instrumentation
  • Plethysmography / methods*
  • Prospective Studies
  • ROC Curve
  • Shock / blood
  • Shock / physiopathology


  • Oxygen